[Feature] Make GAE return a 'value_target' entry

[vmoens]

Description

We return the 'value_target' in the GAE output keys to allow the value error to be computed without pain.

[codecov]

Codecov Report

Merging #741 (77e49fd) into main (f0c8c59) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 77e49fd differs from pull request most recent head 3854c81. Consider uploading reports for the commit 3854c81 to get more accurate results

@@            Coverage Diff             @@
##             main     #741      +/-   ##
==========================================
- Coverage   88.72%   88.71%   -0.01%     
==========================================
  Files         123      123              
  Lines       20725    20726       +1     
==========================================
- Hits        18388    18387       -1     
- Misses       2337     2339       +2     

Flag	Coverage Δ
habitat-gpu	25.11% <0.00%> (-0.01%)	⬇️
linux-brax	29.51% <0.00%> (-0.01%)	⬇️
linux-cpu	85.22% <100.00%> (+<0.01%)	⬆️
linux-gpu	86.16% <100.00%> (+<0.01%)	⬆️
linux-jumanji	30.25% <0.00%> (-0.01%)	⬇️
linux-outdeps-gpu	71.90% <100.00%> (+<0.01%)	⬆️
linux-stable-cpu	85.08% <100.00%> (+<0.01%)	⬆️
linux-stable-gpu	85.80% <100.00%> (+<0.01%)	⬆️
linux_examples-gpu	43.09% <0.00%> (-0.01%)	⬇️
macos-cpu	84.90% <100.00%> (-0.01%)	⬇️
olddeps-gpu	75.87% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
torchrl/objectives/value/advantages.py	86.71% <100.00%> (+0.09%)	⬆️
test/test_trainer.py	98.23% <0.00%> (-0.33%)	⬇️

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